The present invention relates to a method for controlling ignition timing of an internal combustion engine such as an automotive engine through a learning operation, and more particularly to the method for checking a learning correcting quantity of the learning operation at starting of the engine.
In a learning control system for correcting the ignition timing, a correcting quantity is gradually increased or decreased in accordance with an occurrence or absence of knocking. The correcting quantity is usually stored in a memory provided with a backup device so that when an ignition switch is turned on, proper ignition timing can be quickly determined by using the correcting quantity obtained through a learning operation before stopping the engine. Thereafter, when conditions for the learning operation are satisfied, the learning operation is continued to change the correcting quantity in an advance or a retard direction.
However, while the engine stops, the vehicle may be refueled with gasoline having a lower octane number than that used before stopping the engine. In such a case, the ignition timing must be greatly retarded when starting the engine. On the other hand, in a high engine speed range, since it is difficult to detect knocking because large mechanical oscillations generated in the engine are mixed with the knocking, the feedback correction and the learning correction are not performed. Therefore, if the engine speed is in a high engine speed range at starting the engine before the learning correction operation starts, the ignition timing is inevitably determined to be excessively advanced based on the learning correcting quantity for the high octane gasoline used before refueling. Furthermore, the ignition timing is kept advanced because the learning correction is not carried out in the high engine speed range. As a result, the occurrence of the knocking increases, which will cause damage to the engine. Thus, it is preferable to check whether the stored learning correcting quantity is appropriate for that at starting the engine or not.
Japanese Patent Application Laid-Open 61-164076 discloses a control system where correcting quantities dependent on a difference between a basic ignition timing and an actual ignition timing during the feedback operation are stored in a memory to correct the basic ignition timing while the feedback operation is stopped.
However, in the above-described system, the learning correcting quantity is not checked, so that the learning correcting quantity stored in the memory before the stopping of the engine is used in the high engine speed range after restarting the engine. Therefore, if the octane number of the gasoline changes when refueling, knocking frequently occurs until the learning correcting quantity is updated.